Re-oxygenation after radiation has been observed with several techniques, but the detailed mechanism(s) is still unknown, and the timing and degree of post-radiation pO2 changes depend on tumor type. In this study, we monitored pO2in two comparable murine tumors after 20 Gy radiation, and measured tumor blood perfusion in both tumors using dynamic Dg-DTPA enhanced MRI before and at signi ficant points in the time course of post-radiation pO2 changes. The pO2after radiation showed a two-phase effect. The minimum pO2 values were obs erved at 6 hours (MTG-B) or 24 hours (RIF-1) after radiation. Maximum pO2 values were observed 48 hours (MTG-B) or 72 hours (RIF-1) after radiation. Tumorblood perfusion was markedly dropped at the time of minimum pO2 (6 or 24 ho urs after radiation) and then recovered to the initial level at the time of maximum pO2 48 or 72 hours after radiation. Our results suggested that tumor blood perfusion is one of the main factors in re-oxygenation.